There have been broadly employed radiographic images such as X-ray images for diagnosis of the conditions of patients in the medical sites. Specifically, radiographic images using an intensifying-screen/film system have achieved enhancement of speed and image quality over its long history and are still used on the scene of medical treatment as an imaging system having high reliability and superior cost performance in combination. However, these image data are so-called analog image data, in which free image processing or instantaneous image transfer cannot be realized.
Recently, there appeared digital system radiographic image detection apparatuses, as typified by a computed radiography (also denoted simply as CR) and a flat panel detector (also denoted simply as FPD). In these apparatuses, digital radiographic images are obtained directly and can be displayed on an image display apparatus such as a cathode tube or liquid crystal panels, which renders it unnecessary to form images on a photographic film. Accordingly, digital system radiographic image detection apparatuses have resulted in reduced necessities of image formation by a silver salt photographic system and leading to drastic improvement in convenience for diagnosis in hospitals or medical clinics.
The computed radiography (CR) as one of the digital technologies for radiographic imaging has been accepted mainly at medical sites. However, image sharpness is insufficient and spatial resolution is also insufficient, which have not yet reached the image quality level of the conventional screen/film system. Further, there appeared, as a digital X-ray imaging technology, an X-ray flat panel detector (FPD) using a thin film transistor (TFT), as described in, for example, the article “Amorphous Semiconductor Usher in Digital X-ray Imaging” described in Physics Today, November, 1997, page 24 and also in the article “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor” described in SPIE, vol. 32, page 2 (1997).
In order to convert radiation to visible light, it is employed a scintillator panel made of an X-ray phosphor which is emissive for radiation. The use of a scintillator panel exhibiting enhanced emission efficiency is necessary for enhancement of the SN ratio in radiography at a relatively low dose. Generally, the emission efficiency of a scintillator panel depends on the phosphor layer thickness and X-ray absorbance of the phosphor. A thicker phosphor layer causes more scattering of emission within the phosphor layer, leading to deteriorated sharpness. Accordingly, necessary sharpness for desired image quality level necessarily determines the layer thickness.
Specifically, cesium iodide (CsI) exhibits a relatively high conversion rate of X-rays to visible light. Further, a columnar crystal structure of the phosphor can readily be formed through vapor deposition and its light guide effect inhibits scattering of emitted light within the crystal, enabling an increase of the phosphor layer thickness (refer to Patent document 1).
However, since the use of cesium iodide (CsI) alone exhibits a relatively low emission efficiency, so that there is proposed a mixture of CsI with sodium iodide (NaI) at any mixing ratio which is deposited on a substrate via vacuum evaporation as sodium-activated cesium iodide (CsI:Na); or recently, there is proposed a mixture of CsI with thallium iodide (TlI) at any mixing ratio which is deposited on a substrate via vacuum evaporation as thallium-activated cesium iodide (CsI:Tl), followed by subjected to annealing at 200 to 500° C. as a heat-treatment to improve visible conversion efficiency, and the resulting is used as an X-ray phosphor (for example, refer to Patent document 2).
It was found the following by the investigation of the present inventors: since the activator has a different crystal structure from the matrix phosphor compound, it will produce a problem of deteriorated sharpness caused by disorder of columnar crystal structure when the density of activator becomes high (refer to Patent document 3).